build(Makefile): add clean target and refactor the overall structure

[hyperfinitism]

Changes

• Introduce file-target rules for build artifacts (rafwne-enclave.eif, target/release/rafwne-proxy, target/release/rafwne-client) so Make can skip unnecessary rebuilds via timestamp checks; PHONY aliases (build-enclave, build-proxy, build-client) delegate to these file targets
• Add prerequisites to run-* targets (run-enclave → rafwne-enclave.eif, run-proxy → target/release/rafwne-proxy, run-client → target/release/rafwne-client + root.pem) so that builds are triggered automatically when needed
• Replace cargo run with direct binary invocation (./target/release/...) in run-proxy and run-client to avoid redundant Cargo dependency resolution at runtime
• Add clean target (cargo clean, Docker image removal, rafwne-enclave.eif / root.pem cleanup)
• Add .DELETE_ON_ERROR to prevent partial/stale artifacts from surviving failed recipes
• Set .DEFAULT_GOAL := help explicitly
• Refactor help target to auto-generate descriptions from inline ## ... comments on each target
• Move from a single top-level .PHONY declaration to per-target .PHONY for maintainability
• Remove blanket @ silencing from recipe commands to aid debugging; retain @ only on cosmetic echo / printf lines
• Refactor terminate-enclave to use jq --argjson instead of interpolating Make variables inside a jq filter string
• Organize targets into commented sections (Help / Clean / Setup / Build / Run / Terminate) for readability

Notes

No functional changes to build outputs or runtime behavior.